Methods of the type in question are nowadays used on a large scale for transmitting voice, audio, video and/or payload data beyond network boundaries, for example between internal and public data networks. When transmitting data via internet protocol networks (IP networks), problems repeatedly occur when an address translation takes place at a network boundary using Network Address Translation (NAT). NAT is of fundamental importance to internet technology for various reasons in this connection. Among other things, various types of access security in the form of a firewall as well as error tolerance and high availability are supported. Basic network administration functions are also simplified.
Since the address space provided in the founding years of the internet will in the foreseeable future no longer be sufficient for the allocation of IP addresses, with the expansion of internal and highly complex data networks in particular demanding more and more IP addresses, NAT is primarily used to outwardly conceal the internally used IP addresses, however. This makes administration of internal networks easier on the one hand and reduces costs on the other since fewer public IP addresses that are subject to a cost have to be outwardly used than are required internally. A large number of terminals may thus be mapped in an internal network onto a single public IP address by varying the port address of the public IP address.
With NAT, the IP header of an IP data packet is modified before the packet is sent. The internal IP address, including port number, is replaced by a public IP address with a different port number. A NAT host stores the assignment (mapping) of internal IP address to the public (external) IP address. If the NAT host accordingly receives an IP data packet, it maps the public (external) IP address to the internal IP address again. The NAT host is to be understood in this connection as meaning a computer that connects two networks together and on which corresponding software (NAT engine) handles the address translation.